It is common for circuit breakers to employ thermal and magnetic elements which trip the circuit breaker under various overcurrent conditions. The thermal component typically includes a bimetallic element which responds to relatively long duration overload conditions to trip the breaker when a specified current level is exceeded for a period of time. In a typical thermal trip unit, at least a portion of the current flowing through the breaker is channeled through the bimetallic element. The ohmic resistance of the bimetallic element causes it to generate heat in proportion to the square of the level of current flowing through the breaker. As the bimetallic element becomes warmer, it bends and, when it reaches a predetermined temperature, engages a trip mechanism. The trip mechanism releases a latch which holds the breaker contacts closed. When this latch is released, the breaker contacts open, typically responsive to a relatively strong force.
A typical magnetic tripping element includes an armature which is attracted by a magnetic field generated by a relatively high magnitude overcurrent flowing through the breaker. This magnetic field is concentrated by a magnetically permeable yoke which surrounds the conductor through which the current flows. When the armature is attracted to the yoke, it also engages the trip mechanism causing the circuit breaker to open. Both thermal and magnetic structures are used in a typical circuit breaker trip unit to enable the breaker to be tripped on a relatively low overload condition having a long duration and to trip quickly in response to a relatively high overcurrent condition.
It is desirable to be able to configure and adjust the current levels and current durations which cause a breaker to trip in order to customize the circuit breaker to a particular application. Typically, thermal and magnetic structures in circuit breaker trip units may be adjusted to accommodate a relatively narrow variation in current magnitude and current flow duration. These adjustments are made by changing the distance between the bimetallic element and the trip bar for a thermal element and by adjusting the separation between the armature and the yoke for a magnetic trip element.